1. A completely ignorant, childish person with no manners.
2. A person who feels justified in their callow behaviour.
3. A pubescent kid who thinks it's totally cool to act like a moron on the internet, only because no one can actually reach through the screen and punch their lights out.

By definition it's a "rotter" or a despicable person. In reality in English it's a colloquialism that affords itself to a more a luckier fellow. Much like a 'bugger' whilst the roots of the word are offensive, the actual use is endearing. A lucky 'git', a wealthy 'git' and a sodding 'git'... when used right is still a mate. A friend.

It's more positive these days.

The origins are from the word 'geat' which were a Scandinavian people living in Götaland, land of the Geats, currently within the borders of modern Sweden. The name of the Geats lives on in the Swedish counties of Västergötland and Östergötland, the Western and Eastern lands of the Geats, as well as in many toponyms.

From the english dialect "Git" or "Get" meaning "Gate". When using foundry sand to cast iron objects the access point for the metal, that is the funnel shape that lets the molten iron into the sand-mould was called the gate. This gate was of no use and would have to be removed from the final item and the item carefully finished to remove any blemishes caused by the gate. Thus the term "Useless gate (git)" was coined.